The difference between forum vs personal communication is the visual cues, but even then what one says and what another hears can be disastrously different things.

Combine this with the tendency of instant communication, where your fingers type your flow of thought without filters, and there is no cooling off period where you go do something else, come back and re-read your response, and do a 2nd and 3rd revision before sending, and you have a natural setup for miscommunication and escalation.